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Forms of Corrosion (Pitting Corrosion (Condition for Pitting (Presence of…

- - - - Pitting factor = P/d where P = deepest pit length and d = penetration thickness average of uniform corrosion
  - - - For Iron/ carbon steels
        
        Cl⁻, Br⁻, I⁻, ClO₄⁻, SO₄⁻ contaning FeCl₃
      - For Stainless steel
        
        Cl⁻, Br⁻, SCN⁻
      - For Aluminium
        
        Cl⁻, Br⁻, I⁻, NO₃⁻, ClO₄⁻, SCN⁻
  - - - Rate of corrosion inside pits is significantly high due to autocatalytic process
- - - - Impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain boundary areas.
  - - - Diffusion rate of carbon atom is too low
      - Carbide precipitation along grain boundaries cannot occur
    - - Carbide M₂₃C₆ precipitate along grain boundaries leading to formation of chromium depleted zone adjacent to grain boundaries
        
        Depletion of chromium in the grain boundary regions results in intergranular corrosion of stainless steels
        
        The chromium helps to procrastinate nature's attempts to combine the pure iron with oxygen and water to form rust.
        
        M mostly Cr and the rest is Fe and Mo
        
        The chromium-depleted zone is much less corrosion resistant than the surrounding grains
        
        due to
        
        Formation of an unfavorable area ratio consists of a large cathode and a small anode
        
        Result in microscopic localized galvanic attack
        
        IGC
      - Heating of austenitic stainless steels in this temperature region will results in sensitized stainless steels (sensitive to IGC).
    - - Dissolution of carbide
  - - - 950°C<T<1450°C
        
        Chromium carbide precipitates
      - 1450°C<T<2250°C
        
        Niobium carbide precipitates
        
        Chromium carbide dissolves
      - T>2250°C
        
        Niobium carbide dissolves
        
        Chromium carbide dissolves
      - 70°C<T<950°C
        
        No reaction
    - - If the material is reheated to critical temperature, localized sensitization is taking place in that place
        
        Failure can occur by conventional IGC in the narrow sensitized region
    - - Heating above 815°C to dissolve chromium carbide and to form Nb/ Ti carbide at the same time
  - - - Lower acidity and less oxidizing conditions will reduce susceptibility of IGC
    - - Heating the alloy above 815°C to re-dissolve chromium carbide and followed by rapid cooling (often by water immersion)
    - - Chromium carbide will form but material is not sensitive to IGC
- - - - Greatest corrosion on the more active metal near the junction
  - - - surrounding moisture, soil
        
        Greater corrosion near seashore than in dry rural atmosphere
        
        Condensate near seashore contains salts
        
        More corrosive
    - - Ratio of the cathodic to anodic areas
        
        Significant with a large cathode and a small anode
        
        because
        
        Corrosion taking place with high current density
        
        Corrosion = Current density (I/A)
    - - Based on EMF/ Galvanic series position
        
        Significant if the potential between two metals/ alloys ≥ 250 mV
  - - - Large cathode and small anode
- - - - Corrosion site
  - - - Increase concentration of metal chloride
        
        Hydrolysis of ferrous ions
        
        Fe+2Cl⁻ + 2H₂O = Fe(OH)² + 2HCl
        
        Fe(OH)² = insoluble hydroxide (rust)
        
        HCl= free acid
    - - Cathodically protects the external surface
        
        Attack is localized within shielded areas
        
        Crevice corrosion
  - - - To close crevices
    - - Avoid sharp corners and stagnant areas